Hearing restoration or compensation devices, commonly known as hearing aids, provide a tremendous benefit to a patient with congenital hearing loss or whose hearing has deteriorated due to age, genetics, illness, or injury. There is a wide variety of commercially available devices that can be worn externally or can be implanted within the body of the patient.
In general, the element that stimulates the patient's anatomy may be referred to as a transducer. For safety, reliability and diagnostic reasons, it may be desirable to occasionally measure the impedance of the transducer. For this document, the impedance may be considered to be electrical resistance as a function of frequency; the common term “resistance” usually refers to the DC condition, or a frequency of zero.
In the case of a cochlear implant, the electrode is the transducer. Industry standards call for a maximum charge per electrode area of stimulus to avoid damaging the anatomy with excessive charge density. Since the charge density is charge per electrode area, the electrode area is known to the manufacturer. The remainder of the electrical impedance may be a function of the patient's anatomy and may vary depending on the distance between the source and return electrodes. The impedance may also vary with the patient's physiology and hydration levels, which may change over time. Without an accurate impedance measurement, the manufacturer or clinician may err on the conservative side when programming the device to ensure that the device does not cause anatomical damage in the worst case, or lowest impedance, circumstance. This may lead to overly conservative device settings and may consequently unduly limit the efficacy of the device.
In the case of a middle ear implant, the transducer may deliver mechanical vibrations to the patient's anatomy instead of direct electrical stimulus. For these devices, the transducer may include an electrical coil coupled to a magnet on the ossicular chain, or may include a piezoelectric transducer (PZT) affixed to the anatomy, or any other technology for delivering mechanical vibrations to the patient's anatomy. In any of these cases, the transducer could be damaged during shipping or during the surgical process of implantation. The transducer may also fail after the surgical process is complete. If the implantable medical device had a sufficiently accurate impedance measurement diagnostic capability, the clinician could possibly determine which transducer is damaged and could possibly recommend a surgical intervention to replace the damaged transducer.
For some devices, the transducer, as well as the sensor/microphone, may have removable connectors that tether it to a central housing. Such removable connections may be particularly desirable if the transducer and sensor/microphone last longer than the battery in the device, so that the housing may be removed to replace the battery, without removing the transducer and sensor/microphone. Despite these benefits, the removable electrical connections may come loose over time and may disconnect from the processor in the housing. An impedance measurement may be able to detect this condition as well.
Accordingly, there exists a need for measurement of the impedance of the transducer in a hearing aid.